This invention concerns protective eyewear, particularly protective or noncorrective eyewear with decentered optics.
A serious obstacle to the more ubiquitous use of protective eyewear (such as sunglasses and goggles) is that protective lenses can distort vision. This distortion has been thought to be caused by unwanted dioptric power or prismatic effects in the lens, which has been particularly severe in protective lenses that are designed to curve around the eye to the side of the head (xe2x80x9cwrapxe2x80x9d) and/or tilt inward toward the cheekbone (pantoscopic tilt). Although wrap and tilt are aesthetically pleasing, and can provide superior physical protection of the eye, they can also cause the normal line of sight of the eye to strike the lens surface at an angle. This relationship has caused optical distortion that is distracting to the wearer, and presents a serious problem to persons who require precise visual input, such as athletes, pilots and surgeons. This distortion can also be problematic when performing even more common tasks.
The prior art is replete with examples of efforts to overcome optical distortion in protective eyewear. At first such lenses were made with concentric surfaces having no optical center or optical centerline, but the lenses had inherent minus power (which was considered undesirable), and excessive prismatic distortion along the line of sight and peripherally. Later lenses were made plano (zero power), and centered with the optic center at the geometric center of the lens aperture or eyewire, but the plano lenses were found to induce base out prism along the line of sight, and had poor peripheral optics.
Rayton""s U.S. Pat. No. 1,741,536 (issued in 1929 to Bausch and Lomb) disclosed a protective goggle in which the front and back surfaces of the lenses were defined by two spheres of different radii having offset centers. An optical axis through the centers of the spheres was spaced from, and oriented parallel to, a line of sight. This optical configuration provided a tapered lens, in which the lens thickness gradually decreased from the optical center toward the edges. Maintaining the line of sight parallel to the optical axis helped neutralize the distortion that would otherwise be caused by wrapping the lenses laterally with respect to the eye.
In the 1980s, the Foster Grant Company sold dual lens Eyeguard protective eyewear, which held a tapering spherical lens in front of each eye with both wrap and pantoscopic tilt. The optical axis of each lens was horizontally and vertically spaced from, and maintained parallel to, the normal line of sight. This same concept was again claimed many years later in U.S. Pat. Nos. 5,648,832 and 5,689,323, which issued to Oakley, Inc. The parallel relationship between the optical axis and normal line of sight was found to be partially successful in minimizing optical distortion caused by wrap and pantoscopic tilt, but these lenses still had undesired peripheral performance, with prismatic effects that produced yoked and vergence demands.
U.S. Pat. Nos. 4,271,538 and 4,964,714 disclosed a similar position of the optical centerline in safety spectacles, where the optical centerline was horizontally and vertically displaced from, and parallel to, a normal line of sight. However, the ""538 patent itself acknowledged that this relationship left a prismatic imbalance between the right and left eyes, that imposed a vergence demand on the eyes. Like the Foster Grant and Oakley eyewear, these lenses also suffered from undesired peripheral performance, with prismatic imbalance between the eyes that produced yoked and vergence demands.
In the correction of ordinary refractive errors such as myopia and hyperopia, the optical axis of a corrective lens may be slightly tilted from the normal line of sight. A slight downward decentration is commonly used in lenses that have pantoscopic tilt, to help keep the optical axis of the lens directed through the center of rotation of the eye. In a lens having 5-10 degrees of pantoscopic tilt, for example, the optical axis is often shifted about 3 mm below the normal line of sight. A deliberate decentration of a corrective lens may also be needed to compensate for misalignment of the eyes (such as phorias and tropias).
Decentered lenses may be manufactured by cutting a lens blank away from the geometric center of the lens blank. However, the periphery of an injection molded lens blank often includes optical irregularities, and those irregularities are incorporated into a lens which is cut from the edge of the blank. If the decentration is large, the dimensions of the lens must be small so that it can be cut from the lens blank. Alternatively, a larger lens blank can be used, but this solution leads to an inefficient use of large (and relatively more expensive) lens blanks. This problem is particularly acute for lenses manufactured in large quantities, where an incremental increase in the size of the lens blank can significantly increase the manufacturing cost.
It is an object of one embodiment of this invention to minimize optical distortion in protective and noncorrective lenses.
An object of an alternative embodiment of the invention is to provide a more efficient manufacturing method for decentered lenses.
The foregoing problems are addressed by the low minus power lenses of the present invention, which have an optical axis that is angularly deviated at a sufficient angle away from parallel with the line of sight to minimize prismatic distortion, both along a line of sight and peripherally in the field of view. This lens is particularly adapted for protective, non-corrective eyewear in which the lens is mounted in a tilted orientation with respect to the line of sight. The optical axis of the lens is angularly deviated in a direction generally opposite the direction that the low minus power lens is to be tilted, which has surprisingly been found to minimize optical distortion in the lens. This design contravenes the longstanding teaching in the art (since the 1929 Rayton patent), that the optical centerline in non-corrective lenses must be maintained as parallel as possible to the line of sight. The present design has surprisingly been found to reduce both yoked and vergence demands, as well as astigmatic blur, in eyewear made with such lenses.
This invention has taken advantage of the inventors"" recognition that tilting a non-corrective plano lens toward the face induces prism base in the direction in which the lens is tilted. For example, when the inferior edge of a lens that is mounted with pantoscopic tilt is inclined toward the face, base down prism is induced. In accordance with the present invention, the optical axis of a low power lens is deviated generally superiorly, in a direction substantially opposite the direction of the prism induced by the tilt, to offset the tilt induced prism. Similarly, a lens mounted with lateral wrap (temporal edge inclined towards the face) has been found to induce base out prism, which is offset in accordance with the present invention by angular deviation of the optical axis of a low power lens in a generally nasal direction. Lenses that are to be mounted with pantoscopic tilt and lateral wrap may therefore have an optical axis that is deviated both superiorly and nasally to minimize the prism induced by the tilt. Optical compensations for other directions of lens tilt can similarly be achieved by deviating the optical axis generally away from the inward tilt of the lens.
The low power lens may have any amount of minus power, up to that for a concentric lens for a given base curvature. Low power lenses may, for example, have more minus power than xe2x88x920.005, for example more than xe2x88x920.01 or xe2x88x920.02 and particularly in the range of xe2x88x920.01 to xe2x88x920.12 diopters, for example xe2x88x920.04 to xe2x88x920.09 diopters. The use of such low minus power is contrary to the teaching of the art, which has been that plano (non-power) lenses were essential for protective eyewear that does not distort the vision of the wearer. By going against the teaching of the art, the inventors have achieved a number of advantages. The low power lenses have less taper, and can be thinner than zero power lenses. The reduction in taper produces a corresponding reduction in peripheral prism that would otherwise be induced by the excessively non-parallel surfaces of the plano lenses. Thinner lenses also provide better physical stability, improved optical quality, lower weight, and more uniform light transmission than plano lenses. A physiologic advantage of the low minus lens is that it better matches the accommodative posture of athletes and other persons engaged in highly visually demanding and/or stressful activities.
The versatility of the present invention allows it to be applied to a wide variety of lenses having different degrees of lateral wrap, pantoscopic tilt, powers, center thicknesses, and lens surface curvatures, because the prism induced by the tilt (xe2x80x9cprism by tiltxe2x80x9d) can be neutralized by altering a number of these factors. Hence a lens with substantial pantoscopic tilt may have a larger separation between the apex and the line of sight, and a corresponding increase in prism by tilt. This prism can be reduced by one or more of a combination of parameters, such as increasing the angle of deviation between the line of sight and optical axis, increasing the minus power of the lens, or reducing the base curvature of the lens.
The present invention is particularly well adapted to high base lenses, which are at least base 4 lenses, for example a base 6-9 lens, and a base 6 lens in particular embodiments. The lens is also particularly suitable for use in dual lens eyewear, with lenses having a center thickness of about 1-3 mm (for example about 1.5-2.25 mm), a power of about xe2x88x920.01 to xe2x88x920.12 (particularly about xe2x88x920.04 to xe2x88x920.09), a pantoscopic tilt of 3-20 degrees, and lateral wrap of 5-30 degrees. In particular embodiments, the lens is a 6 base lens with a center thickness of about 1.6 mm, a power of about xe2x88x920.045 diopters, and the tilted orientation of the lens includes lateral wrap of about 15 degrees, a pantoscopic tilt of about 12.5 degrees, and the angular deviation between the optical axis and the line of sight (or a parallel to the line of sight) is about 22-23 degrees nasally and 18-19 degrees superiorly.
The lenses of the present invention may be spherical, cylindrical, toroidal, elliptical, or of other configurations known in the art. However, the particularly disclosed embodiment is a spherical lens in which a substantially spherical anterior surface substantially conforms to a first sphere having a first center, and a substantially spherical posterior surface substantially conforms to a second sphere having a second center. The radius of the first sphere is greater than a radius of the second sphere, so that a lens thickness tapers away from an optical center of the low power lens (which may be on or off the lens), and an optical axis extends through the first and second centers of the spheres and the optical center of the lens. This optical axis is angularly rotated nasally and superiorly away from the parallel with the line of sight (to compensate for lateral wrap and pantoscopic tilt), to a sufficient extent to substantially offset prism induced by tilt (for example reducing prism by at least 25%, 50%, 75%, or 100%). This angular deviation provides a lens having a broad spectrum of improved optical properties, including reduced prism (to substantially zero along a functional line of sight in optimal embodiments), reduced astigmatic blur along both the line of sight and peripherally, and reduced yoked and vergence demands.
Another independent aspect of the invention is that the lens may be cut from a lens blank with the A line of the lens angled at a non-zero acute angle to the equator of the lens, from which the lens vertically tapers symmetrically. In particular embodiments, an optical center of the lens blank is displaced along the equator of the lens, away from the geometric center of the blank, although in other embodiments the optical center may also be displaced in the direction of the vertical meridian of the lens (which vertically bisects the equator), so that the optical center is not positioned along the equator, and in particular is not on either the equator or vertical meridian. In some particularly advantageous embodiments, the optical center is not on the lens blank at all. The optical center may be displaced along a line coincident with the equator, and under certain conditions is more advantageously vertically displaced from the equator so that the optical center is on a line that extends through the geometric center of the blank at an acute, non-zero angle.
The invention includes methods of manufacturing lenses, using these and other blanks, in which an optically corrected, protective lens is to be mounted in a tilted orientation in a frame. The lens is cut from a lens blank at a position such that an optical axis of the lens is to be horizontally and vertically displaced from a line of sight, and angularly deviated to the line of sight at an angle sufficient to offset at least some (and preferably substantially all) of the prismatic distortion along the line of sight, and most of the prismatic distortion in the periphery, which has been introduced into the lens by the tilted orientation. Hence, the lens outline is deliberately displaced from the equator and angled in both a horizontal and vertical direction with respect to the equator and vertical meridian of the blank, such that the A line of the lens is oriented at a non-zero angle with respect to the equator. When the lens is rotated around or with respect to the optical center, so that the position of the optical center in relationship to the lens shape is not changed, the optical and geometric characteristics of the lens are preserved. This method allows the lens blank to be used more efficiently, and permits flexibility in positioning the lens in a position that avoids injection mold gate irregularities and peripheral plastic distortions inherent in many lens blanks.